Geochemistry of coexisting depleted and enriched Paringa Basalts, in the 2.7 Ga Kalgoorlie Terrane, Yilgarn Craton, Western Australia: Evidence for a heterogeneous mantle plume event
Section snippets
Introduction and scope
Neoarchean greenstone terranes globally record a number of volcanic associations. Komatiites and associated basalts are generally interpreted as products of anomalously hot mantle plumes, erupted in ocean basins or continental margins (Arndt, 1994, Arndt, 2008, Hollings et al., 1999). Where there is a combination of field, trace element, and isotopic evidence for the absence of crustal contamination, basalts are tholeiitic with near-flat REE patterns over a range of REE abundances, and both
Regional geological setting
The Neoarchean (2800–2600 Ma) Eastern Goldfields Superterrane (EGST) forms the easternmost portion of the Yilgarn Craton (Myers, 1997, Barley et al., 2003, Barley et al., 2008, Krapez, 2006). This Superterrane comprises elongated, narrow to arcuate greenstone belts, composed of deformed and metamorphosed volcanic and sedimentary rocks, intruded by numerous granitoids and high-level intermediate to silicic porphyries. According to Barley et al., 1998a, Barley et al., 1998b, Barley et al., 2002,
Volcanic facies and textures
The Paringa Basalt is part of the Upper Basalt Unit of the Kambalda Sequence (Fig. 2). It is composed of fine- to medium-grained massive or pillowed mafic flows. The massive lavas most likely represent tabular or sheet flows. Medium- to coarse-grained gabbroic rocks are present in both the upper and lower parts of the Paringa Basalt sequence and may represent the central part of thick flows. Some flows contain varioles (up to 4–5 mm in diameter), commonly coalesced, giving a bleached appearance
Analytical methods
Abundances of major and selected trace elements were determined at Geoscience Australia, Canberra, using wavelength-dispersive X-ray Fluorescence Spectrometry (XRF: Philips XRF PW 2404). The concentration of the major elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P and S) was determined on fused disks using methods similar to those of Norrish and Hutton (1969). Precision for these elements is better than ±1% of the reported values. The minor elements Ba, Cr, Cu, Ni, Sc, V, Zr were determined on
Magma series
Based on the Jensen (1976) triangular diagram, depleted Paringa Basalt plots in the high-magnesium tholeiitic basalt (HMTB) field, whereas the enriched counterpart extends across the komatiitic-basalt and HMTB fields (Fig. 5). The enriched Paringa Basalt plots as calc-alkaline on the FeO*/MgO versus SiO2 diagram of Miyashiro (1974; not shown) for distinguishing tholeiitic from calc-alkaline magma series, and as basaltic andesite on the discrimination diagram of Winchester and Floyd (1976; not
Alteration and metamorphism
Given that all rocks in the Kalgoorlie Terrane have undergone sea-floor hydrothermal alteration, poly-phase deformation, and greenschist to amphibolite facies metamorphism, variable element mobility is a possibility. In order to minimize the effects of these secondary processes, samples were carefully selected in the field by avoiding those sites that are most susceptible to chemical changes, such as shear zones, brecciated domains, and areas containing multiple veins and bleaching.
Geodynamic implications and conclusions
Crustally uncontaminated, depleted Paringa Basalts have systematically positive Nb anomalies of within-plate asthenospheric melts and plot along the MORB-OIB array in Th/Yb versus Nb/Yb coordinates, not above the array as for arcs basalts (cf. Pearce, 2008; not shown). Accordingly, the MORB-OIB array was established by the Neoarchean (Wyman and Kerrich, 2009). There is field evidence for synvolcanic extension, textural evidence for submarine eruption, and absence of significant crustal
Acknowledgements
This paper forms part of Nuru Said's PhD project, funded by a University of Western Australia postgraduate scholarship. The whole-rock analyses were generously funded by Professor T.C. McCuaig, Director of the Centre for Exploration Targeting, School of Earth and Environment, who is gratefully acknowledged. Helen Waldron of Genanalysis Laboratory Services and Dr David Champion of Geoscience Australia are acknowledged for the whole-rock geochemical analyses and for ensuring the quality of the
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